Dry Etching
Question
- In a reactive ion etching (RIE) system, describe the four main electron-molecule interactions that occur in the plasma. Which produces the species most responsible for etching?
Answer
(1) Dissociation → radicals: e.g. e + CF₄ → CF₃ + F + e. Produces highly reactive neutral species. (2) Ionisation: e.g. e + Ar → Ar⁺ + 2e. Creates positive ions and additional electrons, sustaining the plasma. (3) Dissociative electron attachment: e.g. e + SF₆ → SF₆⁻. Captures an electron to form a negative ion — removes electrons from the plasma, making it harder to sustain. (4) Electron impact excitation: e.g. e + F → F* + e. Produces excited states that emit light upon relaxation — useful for end-point detection but not a major etch mechanism.
Dissociation produces the most etch-relevant species. In a typical RIE plasma, radicals do most of the etching. Despite the name “reactive ion etching”, ions primarily assist by activating radical formation at the surface rather than directly performing the bulk of material removal.
Question
- Explain how the DC self-bias voltage develops in an asymmetric RIE system with a blocking capacitor. Why is the blocking capacitor essential?
Answer
When the RF voltage goes negative, the driven electrode attracts positive ions from the plasma. These are slow (heavy) and deliver a small positive charge to the electrode. When the RF swings positive, fast electrons flood the electrode, rapidly depositing negative charge. The blocking capacitor prevents DC current flow, so this charge accumulates. Over many cycles, the electrode acquires a net negative DC offset — the self-bias voltage V_dc — which stabilises when the time-averaged electron and ion currents are equal (zero net charge per cycle).
In an asymmetric system (small driven electrode, large grounded electrode/chamber walls), most of the voltage drop appears across the dark space at the smaller electrode, giving a large V_dc there. Ions are accelerated across this sheath with energy ≈ e(V_p + |V_dc|).
Without the blocking capacitor, the electrode would be directly grounded, V_dc would be zero, and the plasma potential would swing symmetrically. Insulating surfaces would accumulate charge with no neutralisation mechanism, leading to arcing and uncontrolled etching.
Question
- What determines the choice of etch gas chemistry? Illustrate with examples for Si, SiO₂, GaAs, and organic materials.
Answer
The key requirement is that the reaction product must be volatile — its vapour pressure must exceed the chamber pressure (typically 10–100 mTorr) so it can be pumped away without redepositing.
Si: fluorine-based (SF₆, CF₄) or chlorine-based (SiCl₄, Cl₂). SiF₄ and SiCl₄ are volatile. SiO₂: fluorocarbon gases (C₂F₆, CHF₃, C₄F₈). The carbon component can also form protective sidewall polymer for vertical profiles. GaAs: chlorine-based (SiCl₄, BCl₃, Cl₂). GaCl₃ and AsCl₃ are volatile. Organics/resist: O₂ plasma. Carbon and hydrogen in the polymer are oxidised to CO₂ and H₂O.
Additionally, CH₄/H₂ is used for III-V and II-VI compounds and magnetic metals — it forms volatile metal-organic (methyl) compounds.
Question
- Explain the difference between physical sputtering and reactive (chemical) etching. How does the etch profile differ in each case, and why does physical sputtering not produce vertical sidewalls?
Answer
Physical sputtering: momentum transfer from an incident ion knocks surface atoms out. No chemical reaction occurs. The sputter yield depends on the angle of incidence, peaking at ~40–60° rather than at normal incidence. This angular dependence means the sidewall (which the ions hit at a glancing angle near the peak erosion rate) is eroded faster than the bottom, producing a sloped profile rather than a vertical one. Trenching can also occur at the base of sidewalls from ions deflecting down the slope.
Reactive etching: the incident species (typically radicals) react chemically with the surface to form a volatile product. Because radicals are uncharged and arrive isotropically, purely chemical etching is isotropic — similar to wet etching. However, ion-assisted reactive etching is anisotropic: ions enhance the reaction rate on horizontal surfaces (which they strike normally) but not on sidewalls (which ions don’t reach), producing vertical profiles.
Question
- What is meant by selectivity in dry etching? Describe how high selectivity is achieved when etching GaAs gate recesses, stopping on a thin AlGaAs etch-stop layer.
Answer
Selectivity is the ratio of etch rates between the target material and the underlying (or adjacent) material. High selectivity means you can etch through one layer and stop precisely on the next.
For GaAs/AlGaAs gate recesses: a mixture of SiCl₄/SiF₄ is used. The fluorine component reacts with the Al in AlGaAs to form AlF₃, which is non-volatile and forms a passivating layer that effectively stops the etch. The ratio of SiCl₄ to SiF₄ can be varied to tune selectivity between 1:1 and 10,000:1. The AlGaAs etch-stop layer can be as thin as 5 nm if the selectivity is high enough.
Question
- List three strategies to minimise dry etch damage in III-V semiconductors. Why is damage more problematic in III-Vs than in silicon?
Answer
(1) Use the lowest ion energy possible — reduce RF power, increase pressure to lower the DC self-bias. (2) Use a high etch rate — if the damaged layer is removed as fast as it forms, net damage is minimised. (3) Avoid inert gas ions (Ar, He as dilutants) — these can only sputter physically, causing deep subsurface damage through trap and vacancy generation.
Damage is more problematic in III-Vs because they cannot be annealed at high temperatures to repair crystal damage (unlike Si, which can be annealed at ~1000°C). III-V compounds decompose or the group-V element desorbs at the temperatures that would be required, so the damage is permanent.
Question
- Describe two in-situ methods used to monitor and control etch depth in RIE. Under what circumstances would each be preferred?
Answer
(1) Optical emission spectroscopy (OES): a spectrometer monitors wavelengths of light emitted by the plasma. When etching through material A to reach material B, the appearance of spectral lines from B’s etch products (or disappearance of A’s products) signals the endpoint. Best when etching through one material to reach a different one.
(2) Laser reflectometry/interferometry: a laser reflects off the etching surface and mask, producing interference fringes as the etch depth changes. The fringe period gives λ/(2n), allowing precise depth measurement. For multilayer structures with different refractive indices, the reflected intensity changes abruptly at interfaces. Best when precise depth control within a single material is needed or when monitoring etch rate in real time.
Question
- In a symmetric electrode RIE configuration, the plasma potential V_p is much larger and the self-bias V_dc is small. What are the advantages and disadvantages of this arrangement compared to an asymmetric one?
Answer
Advantage: the sample can be placed on the grounded electrode. Since V_dc is small, the grounded electrode doesn’t need active cooling to manage heat from ion bombardment, simplifying sample temperature control.
Disadvantage: the large V_p means ions striking the chamber walls have high energy. This causes sputtering of wall material, leading to sample contamination. In an asymmetric system, V_p is small and most of the voltage drop is at the driven electrode only, so wall sputtering is minimal.
Question
- Explain what loading effects and micro-loading effects are in dry etching. Why do they occur?
Answer
Loading effect: the etch rate varies depending on the total exposed (unmasked) area. More exposed area means more material consuming reactive species, depleting them faster — so the etch rate drops. This implies the etch rate is limited by the supply of active species, not by the reaction kinetics at the surface.
Micro-loading: very fine features etch more slowly than wider features on the same wafer, but only below a critical dimension. This occurs because reactive species have difficulty diffusing into narrow, high-aspect-ratio trenches — there’s a transport limitation. The effect is aspect-ratio dependent and becomes particularly significant in through-wafer etching.